Fire detector having unheated thermistors, in particular ntcs, for capturing thermal fluctuations in the region of the inlet openings, and corresponding method

Abstract

A fire detector (M) is configured to capture a fire parameter and to output a fire alarm (AL). It comprises a housing (G) having inlet openings (OF) and thermistors (T1-T4), in particular NTCs, arranged in the region of the inlet openings. The fire detector comprises a control unit (MC) that is connected to the thermistors in order to capture temperature measurement signals (S1-S4). The control unit is characterized in that it is configured to output a warning message (WM) concerning the absence of moving ambient air around the at least one thermistor if the signal bandwidth of signal fluctuations of at least one of the captured temperature measurement signals, preferably all of the temperature measurement signals, falls below a value of 30 mK, in particular 20 mK, preferably 15 mK, for a minimum time (MZ). Causes for the absence of thermal fluctuations may be soiling in the region of the inlet openings or the fact that the inlet openings are covered by a protective cover or by an adhesive tape.

Description

[0001] Description

[0002] Fire detectors with unheated thermistors, especially NTCs, for detecting thermal fluctuations in the area of ​​the inlet openings and corresponding method

[0003] The invention relates to a fire detector, in particular an (optical) smoke detector for detecting at least one fire parameter in the vicinity of the fire detector and for issuing a (fire) alarm in the event of a detected fire. The fire detector has a housing with at least one inlet opening, a sensor system communicating with the ambient air via this inlet opening for detecting a flow in the region of the sensor system, and a control unit. The control unit is connected to the sensor system for evaluating the operational capability of the fire detector. Furthermore, the invention relates to a corresponding method.

[0004] Such fire detectors are known, for example, from EP 1 857 989 Al, from DE 10 2009 000 393 Al, from EP 2 330 577 Al or from EP 2 189 956 Al.

[0005] EP 2 624 229 A1 discloses a fire detector comprising a housing with a measuring chamber for detecting smoke particles. The fire detector has a flow sensor for detecting a flow as a measured variable for evaluating the operational capability of the fire detector, and a suitable evaluation device based thereon. In a preferred design embodiment therein, the flow sensor is designed as a hot-wire flow sensor, the operating principle of which is based on a hot wire cooling due to the flow. The degree of cooling is a function of the flow velocity and the air temperature.

[0006] Based on this, it is an object of the present invention to provide an improved fire detector. A further object of the invention is to provide a fire detector with a more energy-efficient sensor system.

[0007] Finally, it is an object of the invention to provide a method corresponding to the fire detector according to the invention.

[0008] This object is achieved by the subject matter of the independent claims. Advantageous embodiments of the present invention are specified in the dependent claims.

[0009] According to the invention, the sensor system has at least one thermistor arranged in the region of at least one of the inlet openings. A thermistor is a passive component with a temperature-dependent ohmic resistance value. It is preferably an NTC (for Negative Temperature Coefficient Thermistor) or a PTC (for Positive Temperature Coefficient Thermistor), i.e. a thermistor with a negative or positive temperature coefficient. The control unit is connected to the at least one thermistor for the (exclusive) detection of a respective temperature measurement signal. A thermistor is thus a special temperature sensor.

[0010] Further according to the invention, the control unit is configured to output a warning message if the signal bandwidth of signal fluctuations of at least one of the detected, preferably all, temperature measurement signals falls below a value of 30 mK, in particular 20 mK, preferably 15 mK, for a minimum period of time. At least 90% of the detected signal fluctuations, preferably at least 95% of the detected signal fluctuations, lie within this signal bandwidth.

[0011] In other words, the control unit is designed to issue a warning message if the alternating signal component (AC component), i.e. the signal component decoupled from the direct component (DC component), of at least one of the detected, preferably all, temperature measurement signals has a value of less than 30 mK, in particular of less than 20 mK, preferably of less than 15 mK, for a minimum period of time.

[0012] The minimum time shall be in a range of 1 day to 1 year, in particular in a range of 1 day to 1 month, and preferably in a range of 1 day to 1 week.

[0013] The fire detector can have a timer to detect when the minimum time has elapsed. The timer can be an electronic counter module. This can already be integrated in the control unit. Alternatively, the timer can also be implemented as software that runs on a processor-based control unit, in particular a microcontroller. The control unit is set up to initially set the timer counter reading to a start value that corresponds to the minimum time, and then the timer counts down continuously. The control unit then issues a warning message when the timer reaches zero. If the control unit detects signal fluctuations beforehand, the timer is reset to the start value.

[0014] The warning message is issued when there is no moving ambient air around at least one thermistor. The signal fluctuations are caused by thermal fluctuations in the room air that directly surrounds or flows around the respective thermistor. The lack of moving ambient air is an indication of the presence of contamination in the area of ​​the inlet openings or an indication that the inlet openings have been improperly covered with a protective cover or adhesive tape. In this case, the fire detector is no longer operational. The warning message is therefore a message that the fire detector is not operational.

[0015] The core of the invention lies in the use of purely passively operated, i.e. unheated thermistors for the metrological detection of very small signal fluctuations in the respective temperature measurement signal due to thermal fluctuations that are almost always present in the vicinity of the fire detector. If such thermal fluctuations can be detected in the area of ​​the inlet openings, this is a (reliable) indication that the inlet openings are sufficiently permeable for fire detection. At least the inlet openings are not completely dirty or blocked when fluctuations are present, or have been intentionally covered or taped over. The latter is often the case when the fire detector is intentionally covered by a protective hood due to painting work in order to prevent the fire detector from being accidentally painted.The inlet openings are often taped off with masking tape or covered with a rubber glove if such a protective cover is not available.

[0016] Measurement studies have shown that thermal fluctuations, i.e. flow fluctuations, are not completely homogeneous in terms of their temperature distribution. It has also been determined that the (average) signal bandwidth of signal fluctuations that resemble a noise signal in the respective temperature measurement signal, typically in the range around 15 mK, is already many times greater than the pure thermal noise of the thermistor itself. The detection of the very small temperature changes results from minimal heat input into the thermistors or minimal heat dissipation from the thermistors. Since the respective thermistor is not heated, there is no cooling effect that a thermal fluctuation or a flow fluctuation would cause in an otherwise heated flow sensor. The magnitude of the cooling effect would then be a measure of the speed of the flow flowing around the flow sensor.

[0017] Such thermal fluctuations in the vicinity of the fire detector typically result from the opening and closing of doors, from a gust of wind from outside the building hitting windows that are not completely sealed, from air turbulence caused by people or from heaters, air conditioners, fans or blowers turning on and off.

[0018] As previously described, the warning message is suppressed for a period of time corresponding to the specified reference value. It can then be assumed that the fire detector is not excessively dirty if, within this period, an air surge with a temperature slightly different from the temperature of the respective thermistor is occasionally detected.

[0019] The particular advantage of the invention is that no electrical, current- or power-intensive heating of the respective thermistor is required. This advantageously allows the fire detector to be a (solely) battery-powered fire detector. The battery can be used to supply the fire detector exclusively with power.

[0020] A further advantage is that the thermistors used, especially the NTCs, are mass-produced and are considerably cheaper and more compact than dedicated flow sensors.

[0021] Finally, it is a further advantage that these components can be used in a fire detector with excess temperature detection as an additional fire parameter. This advantageously requires fewer components in a fire detector according to the invention compared to fire detectors with a hot-wire flow sensor as the flow sensor.

[0022] In contrast, thermal anemometers, such as hot-wire anemometers, use a sensor element that must be electrically heated and whose electrical resistance depends on the temperature or the cooling effect. The flow around the medium causes heat to be transferred into the flow medium, which changes with the flow velocity. By measuring the electrical variables, the flow velocity can then be determined. However, apart from the disadvantageous high current and energy consumption, the operation of such sensors also requires special electronics that regulate the heating current and amplify the sensor signal.

[0023] If the fire detector according to the invention is an optical smoke detector, in particular a scattered-light smoke detector, a (characteristic) fire parameter is the smoke density, which correlates metrologically with a scattered-light level. The smoke density is monitored for exceedance of a minimum smoke density, i.e., a minimum scattered-light level, metrologically. Alternatively or additionally, the smoke density or the corresponding scattered-light level can also be monitored for an impermissibly rapid increase.

[0024] If the fire detector according to the invention is a thermal detector, a (characteristic) fire parameter is the temperature in the immediate vicinity of the fire detector, which is monitored for exceeding a minimum temperature. Alternatively or additionally, the temperature can also be monitored for an inadmissibly rapid increase. In this case, the control unit of the fire detector is set up to monitor the respective temperature measurement signal for exceeding an excess temperature limit value of in particular 54 ° C and / or for exceeding a temperature rise rate of in particular 5 ° C per minute, preferably 10 ° C per minute, as fire parameters and to issue a fire alarm if a fire is detected.

[0025] The fire detector can also be a combination of an optical smoke detector, particularly a scattered light smoke detector, and a thermal detector. In the simplest case, a fire alarm is triggered as soon as one of the detected fire parameters exceeds an assigned reference value.

[0026] The control unit of the fire detector is, in particular, processor-controlled and preferably a microcontroller. A software program is executed on such a microcontroller by its processor unit. This software program analyzes and evaluates the detected at least one fire parameter, and issues a fire alarm if a fire is detected. It also issues the warning message for the specified minimum time if thermal fluctuations do not occur.

[0027] The fire detector is preferably designed as a point detector. It can be connected to a detector line of a fire alarm control panel, typically with several other fire detectors. Alternatively, the fire detector can be battery-operated.

[0028] According to one embodiment, the control unit is configured to continuously convert the respective temperature measurement signal into a respective digital temperature measurement signal by means of an A / D converter with a predetermined sampling period AP, in particular with a sampling period AP in a range from 0.1 to 10 seconds, preferably in a range from 0.5 to 2 seconds. This enables the typically brief, sporadic occurrence of moving air with thermal inhomogeneities to be reliably detected. The actual A / D conversion time using an A / D converter in a microcontroller is typically less than 10 microseconds, in particular less than 2 microseconds.

[0029] According to a further embodiment, the control unit is configured to first filter the respective acquired temperature measurement signal using a high-pass filter and subsequently detect the signal fluctuations in the respective temperature measurement signal. The cut-off frequency, i.e. the filter frequency, of the high-pass filter is in particular less than 0.1 Hz. This advantageously removes the DC signal component in the respective temperature measurement signal for improved metrological evaluation of the fluctuating AC signal component. The respective resulting high-pass filter signal is thus DC-decoupled. The high-pass filter can be an RC element implemented with discrete components, the output of which is connected downstream of the respective thermistor or downstream of a signal amplifier connected downstream of the thermistor. The aforementioned discrete components can also already be integrated in the control unit.Alternatively, the high-pass filter is a digital high-pass filter.

[0030] According to an alternative embodiment to the previous embodiment, the control unit is configured to first filter the respective detected temperature measurement signal using a high-pass filter, to filter a high-pass filter signal output by the respective high-pass filter using a low-pass filter with a filter time in the range of 1 to 120 minutes, preferably 15 to 60 minutes, and to output it as a respective filter output signal. The cutoff frequency, i.e. the filter frequency, of the high-pass filter is in particular less than 1 Hz, preferably less than 0.1 Hz. The control unit is configured to output the warning message if at least one, preferably all, filter output signals fall below a predetermined limit value of 5 mK, in particular 3 mK, preferably 1.5 mK, instead of the signal bandwidth of the signal fluctuations in the respective temperature measurement signal for a minimum period of time.This enables a more precise evaluation of the temperature measurement signals for the presence of signal fluctuations.

[0031] According to an alternative embodiment to the two previous embodiments, the control unit is configured to first filter the respective detected temperature measurement signal using a high-pass filter, to filter a high-pass filter signal output by the respective high-pass filter using a moving average filter, in particular using a moving quadratic or arithmetic mean filter, with a predetermined filter time in the range from 1 to 120 minutes, preferably from 15 to 60 minutes, and to output it as a respective filter output signal. The cut-off frequency, i.e. the filter frequency, of the high-pass filter is in particular less than 1 Hz, preferably less than 0.1 Hz. In technical terms, such a filter is also referred to as a moving average filter.Another name for the root mean square filter is the root mean square filter, which in technical terms is also called an RMS filter.

[0032] The control unit is configured to issue a warning message if at least one, preferably all, of the filter output signals fall below a predetermined limit value of 5 mK, in particular 3 mK, preferably 1.5 mK, instead of the signal bandwidth of the signal fluctuations in the respective temperature measurement signal for a minimum period of time. If the moving average filter is a moving root mean square filter, i.e. a moving effective value filter or moving RMS filter, then the signal power-related metrological analysis enables particularly precise evaluation of the temperature measurement signals for the presence of thermal fluctuations.

[0033] The previously mentioned high-pass, low-pass and moving average filters are in particular digital filters, preferably stable FIR filters (FIR for Finite Impulse Response), which are implemented as part of a software program on a microcontroller as the control unit. The simplest FIR filter is the classic averaging process by summing individual successive values ​​and dividing by their number. If this is done not in blocks (number 1 to number 10, number 11 to number 20 etc.), but in an overlapping manner (number 1 to number 10, number 2 to number 11, number 3 to number 12 etc.), the result is the moving arithmetic mean. A preferred FIR filter is the averaging process by summing the squares of individual successive values ​​and dividing by their number, followed by root calculation.If this is not done in blocks but in an overlapping manner, one arrives at the moving root mean square (moving RMS).

[0034] According to an advantageous embodiment, the specified limit value is set in a range of 1.5 to 10 times, in particular 1.5 to 5 times, a reference value that has been determined by measurement on a new, as-new, or refurbished, in particular brand-new, fire detector in a flow-free test environment. The measurement is typically carried out at a room temperature of 20 °C ± 2 °C and typically at normal ambient air pressure. The reference value essentially corresponds to the thermal noise or the thermal noise power.

[0035] According to a further embodiment, the control unit is configured to continuously convert the respective temperature measurement signal into the respective oversampled digital temperature measurement signal by means of the A / D converter through a number of oversamples (OVS) in a range from 2 to 4096, in particular with a number of OVS of 64, 256, or 1024. The total time of the oversamples per conversion of a temperature measurement signal into an averaged digital value is less than 50 ms, in particular less than 25 ms, preferably less than 10 ms.

[0036] Oversampling increases the resolution of an A / D conversion. The number of "extra" bits follows the relationship 0 . 5 x Id OVS , where Id is the dual logarithm and OVS is the number of oversamples. With 256 oversamples, the resolution for each converted digital value increases by 4 bits.

[0037] The particular advantage here is that instead of an expensive microcontroller with a 14-bit A / D converter or a 16-bit A / D converter, a cost-effective microcontroller with a 10-bit A / D converter or a 12-bit A / D converter can now be used. In addition, an additional signal amplifier circuit can be omitted. By increasing the resolution for each converted digital value by the exemplary 4 bits, even the smallest temperature fluctuations in the respective temperature measurement signal can be reliably resolved from the thermal noise of the thermistor. According to a further embodiment, the control unit is set up to control the respective thermistor in such a way that it heats up by less than 0.5 mK, in particular by less than 0.1 mK, per measured value acquisition. As a result, the respective thermistor is effectively operated passively with a negligible temperature increase.

[0038] Alternatively or additionally, the control unit is configured to control the respective thermistor in such a way that it absorbs a maximum electrical energy of 10 pj, in particular 5 pj, preferably 0.5 pj, per measured value acquisition. In this case, too, the respective thermistor is effectively operated with a negligible temperature increase.

[0039] According to a further embodiment, the heat capacity of the respective thermistor is dimensioned such that the thermal response time of the respective thermistor is a maximum of 3 s, in particular 2 s, preferably a maximum of 1 s. The "thermal response time" refers to the time period in which the thermistor detects half of a sudden temperature difference when externally applied. This allows even the slightest temperature fluctuations and thus thermal inhomogeneities within the moving air entering the area of ​​an inlet opening or the central interior of the measuring chamber to be detected.

[0040] According to a further embodiment, a fire detection unit, in particular an optical measuring chamber, communicating with the ambient air via the at least one inlet opening is accommodated or configured in the housing for detecting at least one fire parameter. A grille, in particular an insect screen, is arranged in the area of ​​the respective inlet opening and the fire detection unit.

[0041] In a first embodiment, the grid is arranged between the at least one thermistor and the fire detection unit in the region of the respective inlet opening. In particular, the at least one thermistor is arranged radially outwardly of the grid with respect to the main axis of the fire detector. The respective thermistor is located "in front" of the grid when viewed from the outside. Preferably, at least two thermistors are arranged evenly distributed in the circumferential direction relative to the main axis of the fire detector. This advantageously enables particularly reliable monitoring for impermissible covering of the at least one inlet opening.

[0042] In a second embodiment, the at least one thermistor is arranged between the grid and the fire detection unit in the region of the respective inlet opening. In comparison to the previous embodiment, the at least one thermistor is arranged radially inward of the grid with respect to the main axis of the fire detector. The respective thermistor is located "behind" the grid when viewed from the outside. Further preferably, the at least two thermistors are arranged evenly distributed in the circumferential direction with respect to the main axis of the fire detector. If there is an even number of thermistors, these are arranged radially opposite one another with respect to the main axis. This advantageously makes it possible to monitor the grid section assigned to the respective inlet opening for increasing and ultimately unacceptably high contamination. Furthermore, it is also possible to monitor for unacceptable covering of the at least one inlet opening.

[0043] According to a further embodiment, the control unit is configured to start an operating timer of the fire detector when the electrical power supply to the fire detector begins. The control unit is configured to output a covered message, or the warning message together with a covered message, as an indication of impermissible covering of the at least one inlet opening if the signal bandwidth of signal fluctuations in at least one of the detected, preferably in all, temperature measurement signals falls below a value of 30 mK, in particular 20 mK, preferably 15 mK, for a minimum waiting time in a range of 1 hour to 3 days, preferably 1 hour to 24 hours, and if, in addition, an operating time measured value of the operating timer is less than 2 years, preferably less than 1 year.

[0044] The operating timer can be a counter module with non-volatile electronic memory, such as EEPROM, Flash, or FRAM memory. The operating timer can be a separate electronic component or already integrated into the control unit. Typically, a fire detector is supplied with electrical power by a fire alarm control panel via a detector cable or a detector bus when it is inserted into its detector base. In the case of a battery-operated fire detector, the electrical power is supplied when a fire detector battery is inserted or switched on.

[0045] It can be assumed that a fire detector with a short operating time will have minimal contamination around the inlet openings, preventing smoke or fire gases from flowing through the inlet opening into the interior of the fire detector for fire detection. One reason for the lack of fluctuations is most likely that the inlet openings have been covered with a protective cap or, in the absence of such a cap, taped over with adhesive tape to prevent the fire detectors from being painted over during upcoming painting work.

[0046] Advantageously, the masked message or the warning message with the masked message is issued after a fire detector has been reinserted into its detector base or when a fire alarm system with a plurality of fire detectors according to the invention is switched on again, in order to indicate the possible non-operational readiness of a fire detector.

[0047] According to one embodiment, the control unit is configured to output the warning message and / or the masked message visually and / or acoustically directly to the fire detector, such as by means of a flashing LED or a buzzer. Alternatively or additionally, the warning message and / or the masked message can be output to a higher-level control center connected to the fire detector and / or wirelessly via a radio connection to a higher-level control center.

[0048] The object of the invention is further achieved by a method for detecting contamination or an impermissible covering of at least one inlet opening for smoke or fire gas in a fire detector. At least one (unheated) thermistor is arranged in the region of one of the inlet openings. A temperature measurement signal is recorded by the respective thermistor. A timer is started when thermal fluctuations with a signal bandwidth of at least 30 mK, in particular of at least 20 mK, preferably of at least 15 mK, are detected in at least one of the temperature measurement signals.Finally, a warning message is issued in the absence of moving ambient air around the respective thermistor if the (average) signal bandwidth of signal fluctuations in at least one of the detected, preferably all, temperature measurement signals falls below a value of 30 mK, in particular 20 mK, preferably 15 mK, for a minimum period of time. At least 90% of the detected signal fluctuations, preferably at least 95% of the detected signal fluctuations, fall within this signal bandwidth.

[0049] The minimum time shall be in a range of 1 day to 1 year, in particular in a range of 1 day to 1 month, and preferably in a range of 1 day to 1 week.

[0050] In order to avoid unnecessary repetition, the following description will be understood to mean that features and details described in connection with the fire detector mentioned and with its embodiments will of course also apply in connection with and with regard to the corresponding method and vice versa, so that with regard to the disclosure of the individual aspects of the invention, reference will always be made to each other.

[0051] The features and details described for the fire detector according to the invention, in particular the scattered light smoke detector, such as embodiments of thermistors, numerical values, temperature values, time periods, rates of change, etc., also apply to the corresponding method.

[0052] The invention and advantageous embodiments of the present invention are explained using the example of the following figures. In these figures:

[0053] FIG 1 shows an example of a fire detector according to the invention with four thermistors distributed in the area of ​​the inlet opening,

[0054] FIG 2 is a plan view of the fire detector according to the viewing direction II shown in FIG 1,

[0055] FIG 3 is a block diagram of an electronic control unit of the fire detector according to the invention,

[0056] FIG 4 shows the course of an unfiltered temperature measurement signal of a thermistor, arranged in the region of an inlet opening of a fire detector according to the invention,

[0057] FIG 5 shows the course of the magnitude of a high-pass filter signal after filtering the unfiltered temperature measurement signal from FIG 4 in an enlarged view, and

[0058] FIG 6 shows the curve of the magnitude of a filter output signal after filtering the high-pass filter signal from FIG 5 using a low-pass filter in a greatly enlarged view. FIG 1 shows an example of a fire detector M according to the invention with four thermistors T1-T4 distributed in the region of the inlet opening OE. G designates a housing of the fire detector M which comprises a base body GG and a hood H. In between, the fire detector M has, for example, an inlet opening OE which runs completely around a main axis A of the fire detector M. The thermistors T1-T4 shown are part of a sensor system for detecting a flow in the region of the sensor system.

[0059] The hood H shown is supported, for example, by a measuring chamber accommodated in the interior of the housing G and communicating with the ambient air via at least one inlet opening OE. The optical measuring chamber MK here is the preferred embodiment of a fire detection unit MK for detecting at least one fire parameter and is also referred to as a labyrinth. It characteristically has slats (not further designated). The latter are designed, on the one hand, to shield the optical measuring chamber MK from ambient light penetrating via the inlet opening OE and, on the other hand, to be permeable to smoke or fire gases to be detected.

[0060] N denotes an insect screen which prevents insects and the like from entering the measuring chamber MK through the inlet opening OE. The four thermistors T1-T4 are arranged radially outwardly of the screen N with respect to the main axis A. The screen N is therefore located between the thermistors T1-T4 and the slats of the optical measuring chamber MK. If the inlet opening OE is covered, for example, by a protective hood, the thermistors T1-T4 are then aerodynamically shielded from the ambient air.

[0061] In the present example, the optical measuring chamber MK, as is known, has two light-emitting diodes (LEDs) of different colors, each arranged in a scattered light arrangement to form a photosensor PD. A further sensor, e.g., for detecting toxic gases such as carbon monoxide (CO), or a humidity sensor as a comfort sensor, can also be arranged in the measuring chamber MK.

[0062] All of the aforementioned sensors, as well as the light-emitting diode (LED), can be arranged together with an electronic, processor-based control unit (MC) of the fire detector (M) on a circuit board (PCB) of the fire detector (M). The control unit (MC) of the fire detector (M) is preferably a microcontroller that is configured or programmed to issue an alarm (AL) in the event of a detected fire, symbolized by an arrow. In this case, the alarm is triggered when an excessively high scattered light level or an excessively high CO level is detected by a CO sensor in the measuring chamber (MK).

[0063] According to the example of FIGS. 1 and 2, the four thermistors T1-T4 located radially outwards with respect to the main axis A of the fire detector M are arranged evenly distributed in the circumferential direction. The thermistors T1-T4 are preferably NTCs. The latter are extremely robust, very small in their dimensions and also cost-effective. Such NTCs have, apart from the connecting wires, maximum dimensions in their three dimensions in the range 1 mm to 5 mm. Typically, two of the three dimensions are even smaller than 2 mm. This extremely compact design with its low thermal capacity ensures a fast thermal response of a maximum of 3 s, in particular a maximum of 2 s and preferably a maximum of 1 s.Short-term, minor temperature fluctuations in the moving ambient air (thermal fluctuations) then also become apparent at the two electrical connections of the respective NTC in the form of corresponding, metrologically detectable ohmic resistance changes and thus also in an associated electrical temperature measurement signal. The previously mentioned thermistors T1-T4 are therefore set up or intended to output a respective temperature measurement signal. The NTC for the respective thermistors T1-T4 can, for example, have an ohmic resistance value of 10 kΩ, 20 kΩ, 25 kΩ, 50 kΩ or 100 kΩ specified for an ambient temperature of 25°C. The respective NTC is typically connected in series with a resistor (component) which has a constant resistance value. The ohmic resistance value of the resistor (component) preferably corresponds to the resistance value of the respective NTC at 25°C.This series circuit is connected to a constant voltage difference, such as a voltage difference of 3.0 V, typically formed from the voltage difference between a reference potential (ground) and a positive constant supply voltage. The center tap of the series circuit is then fed to an input of an A / D converter. The latter is usually already integrated in a microcontroller MC. Preferably, the electronic control unit or the microcontroller MC is set up or programmed to apply this voltage difference only for the duration of an (ongoing) A / D conversion. This can be done, for example, by electrically controlling a switching element connected in series with this series circuit, such as a transistor. This further minimizes power consumption.

[0064] Alternatively, instead of the ohmic resistor (component), a constant current source can be connected in series with the NTC or, more generally, with the thermistor. Alternatively or additionally, a signal amplifier can be connected between the center tap of the series circuit, whose output signal is connected to the input of the A / D converter.

[0065] According to the invention disclosure, the control unit MC shown is configured or programmed to output a warning message WM if the (average) signal bandwidth of signal fluctuations in at least one of the detected, preferably all, temperature measurement signals S1-S4 falls below a value of 30 mK, in particular 20 mK, preferably 15 mK, for a minimum time MZ. The minimum time MZ is in particular in a range from 1 day to 1 year, in particular in a range from 1 day to 1 month, and preferably in a range from 1 day to 1 week.

[0066] FIG 3 shows a block diagram of an electronic control unit MC of the fire detector M according to the invention, implemented as a microcontroller. A software program PRG is executed on the microcontroller MC by its processor unit in order to control the associated light-emitting diode LED in the case of a connected optical fire detection unit, to receive a sensor signal from the associated photodiode PD and to analyze this using an evaluation program and, if necessary, to output a fire alarm AL. The software program PRG is also executed in order to output a warning message WM in the event of a lack of moving ambient air around the at least one thermistor T1-T4. The absence of thermal fluctuations is an indication of unacceptably high contamination or a blockage of the at least one inlet opening.

[0067] As shown in FIG. 3, the microcontroller MC has, for example, four integrated A / D converters ADC, which convert the temperature measurement signals S1-S4 originating from the four thermistors T1-T4 into a respective digital temperature measurement signal D1-D4. The four digital signals D1-D4 are converted in a functional block (not further designated) by means of a respective high-pass filter HP into a respective high-pass signal F H iF H 4 filtered or converted. In this example, the four high-pass signals F H iF H4 preferably fed to a respective moving root mean square filter RMS. According to the invention, the filter output signals F0I-F04 provided on the output side are then monitored to determine whether at least one of the filter output signals F0I-F04 exceeds a predetermined limit value GW. If this is the case, the counter value of a timer TIMER implemented as software in the microcontroller MC is set to a start counter value corresponding to the minimum time and started to count down. The timer TIMER is reset or restarted with the start counter value each time at least one of the filter output signals F0I-F04 exceeds the predetermined limit value GW again. Finally, according to the invention, the warning message WM for the absence of moving ambient air around the at least one thermistor T1-T4 is output if the timer TIMER reaches the counter value zero.

[0068] Alternatively, the four high-pass signals FHI- F H 4 can also be fed to a respective moving arithmetic mean filter AVS or a low-pass filter TP. The previously described filters HP, RMS, AVS, and TP are digital filters, preferably FIR filters. FZ denotes a filter time for the moving average filters RMS, AVS, or for the low-pass filter TP.

[0069] According to one embodiment, the control unit MC can be connected to an operating timer BZM, which is started when the electrical power supply to the fire detector begins. In this case, the control unit MC is set up or programmed (PRG) to output a covered message COV, or the warning message WM together with a covered message COV as an indication of impermissible covering of the at least one inlet opening, if the signal bandwidth of signal fluctuations in at least one of the recorded, preferably in all, temperature measurement signals S 1 - S 4 falls below a value of 30 mK, in particular 20 mK, preferably 15 mK, for a minimum waiting time WZ in a range of 1 hour to 3 days, preferably 1 hour to 24 hours, and if, in addition, an operating time measurement value BZW of the operating timer BZM is less than 2 years, preferably less than 1 year.

[0070] FIG. 4 shows the curve of an unfiltered temperature measurement signal from a thermistor, arranged in the region of an inlet opening of a fire detector according to the invention. The temperature measurement signal was sampled, for example, with a sampling time of 2 seconds and recorded over a period of just over a month (August 28 to October 2) in a non-air-conditioned room in the applicant's development department. The temperature measurement signal reflects the curve of the ambient temperature with the respective maximum temperatures during the day and the respective minimum temperatures at night. The maximum temperature range for this period was approximately 8 degrees Celsius.

[0071] FIG 5 shows an enlarged view of the magnitude of a high-pass filter signal after filtering the unfiltered temperature measurement signal from FIG 4. The high-pass filter used to decouple the DC component in the temperature measurement signal had a filter time of 25 seconds, which corresponds to a filter frequency of 1 / 25 of a second. A greatly enlarged view of the measurement shows comparatively large signal excursions in the form of signal fluctuations with absolute maximum excursions of approximately 80 mK, which are associated with thermal fluctuations in the ambient air, up until the time the protective cover is put on (see the two boxes labeled "Detector Cover").

[0072] Surprisingly, these signal deflections decrease very quickly when the protective cover is put on. This is due to the shielding effect of the protective cover. As the signal continues, the magnitude of the filtered temperature measurement signal only shows signal fluctuations with deflections of a maximum of approx. 6 mK, which corresponds to a signal bandwidth twice the value of approx. 12 mK, and is thus approx. one order of magnitude lower than in the period before the inlet opening was covered with the protective cover. The high-pass filtering with the absolute temperature differences shown means that the temperature measurement signal remains above the zero line and related to the zero line. The reference symbol B / 2 designates half the signal bandwidth of the signal fluctuations during the period in which the detector is covered.Finally, FIG 6 shows the curve of the magnitude of a filter output signal after filtering the high-pass filter signal from FIG 5 using a low-pass filter with a filter time of 60 minutes in a greatly enlarged view. As FIG 6 shows, the absolute values ​​of the low-pass signal drop drastically from approx. 5 mK before the protective cap is put on to a value of around 1 mK after the protective cap is put on. The reference symbol GW indicates a limit value with an exemplary value of 1.5 mK, which is definitely not exceeded by the absolute values ​​of the low-pass filter signal during the period the detector is covered. This very large and stably reproducible signal difference advantageously enables reliable discrimination as to whether the inlet openings have been covered or are completely dirty.

[0073] In summary, the invention relates to a fire detector configured to detect a fire characteristic and to issue a fire alarm. It comprises a housing with inlet openings and thermistors, in particular NTCs, arranged in the region of the inlet openings. The fire detector comprises a control unit connected to the thermistors for detecting temperature measurement signals. The control unit is typically configured to detect the (average) signal bandwidth of signal fluctuations in at least one of the detected, preferably all, temperature measurement signals below a value of 30 mK, in particular 20 mK, preferably 15 mK, for a minimum period of time. Causes for the absence of thermal fluctuations can be contamination in the region of the inlet openings or covering the inlet openings with a protective cover or adhesive tape. List of reference symbols

[0074] A main axis, axis of symmetry

[0075] ADC A / D converter

[0076] AL fire alarm, alarm

[0077] AVS moving arithmetic mean filter

[0078] B / 2 half signal bandwidth

[0079] BZM operating timer

[0080] OR operating time measurement value

[0081] COV Coverage Report

[0082] DET evaluation unit

[0083] D1-D4 digital temperature measurement signal

[0084] FHI-F H4 High-pass filter signal

[0085] F01-F04 filter output signals

[0086] FZ filter time

[0087] GG base body

[0088] GW limit

[0089] H hood, detector hood

[0090] HP high-pass filter

[0091] LED light transmitter, light-emitting diode

[0092] M fire detector, smoke detector

[0093] MC electronic control unit, microcontroller

[0094] MK fire detection unit, optical measuring chamber

[0095] MZ minimum time

[0096] N Insect screen, net

[0097] OF entry opening, smoke entry opening

[0098] PCB circuit carrier

[0099] PD light receiver, photodiode

[0100] PRG computer program, software

[0101] RMS moving mean square filter

[0102] S1-S4 temperature measurement signal t time, time axis

[0103] T1-T4 thermistor, NTC

[0104] TIMER Timer, timing element

[0105] TP low-pass filter

[0106] WM warning message, contamination message

[0107] WZ minimum waiting time

[0108] Temperature change Temperature

Claims

Patent claims 1. Fire detector for detecting at least one fire characteristic parameter and for issuing a fire alarm (AL) in a detected fire, wherein the fire detector (M) has a housing (G) with at least one inlet opening (OE), a sensor communicating with the ambient air via this inlet for detecting a flow in the area of ​​the sensor, and a control unit (MC), wherein the control unit (MC) is connected to the sensor for evaluating the operational readiness of the fire detector (M), characterized in that - that the sensor system includes at least one thermistor (T1-T4) located in the area of ​​at least one of the inlet openings (OE), in particular at least one NTC, - that the control unit (MC) is connected to at least one thermistor (T1-T4) for detecting a respective temperature measurement signal (S1-S4), and - that the control unit (MC) is configured to issue a warning message (WM) if the signal bandwidth of signal fluctuations falls below a value of 30 mK, in particular 20 mK, preferably 15 mK, for a minimum time (MZ) of at least one of the detected, preferably all temperature measurement signals (S1-S4).

2. Fire alarm according to claim 1, wherein the minimum time is in a range of 1 day to 1 year, in particular in a range of 1 day to 1 month, and preferably in a range of 1 day to 1 week.

3. Fire detector according to claim 1 or 2, wherein the control unit (MC) is configured to continuously convert the respective temperature measurement signal (S1-S4) into a respective digital temperature measurement signal (D1-D4) by means of an A / D converter (ADC) with a predetermined sampling period AP, in particular with a sampling period AP in a range of 0.1 to 10 seconds, preferably in a range of 0.5 to 2 seconds.

4. Fire detector according to one of claims 1 to 3, wherein the control unit (MC) is configured to first filter the respective detected temperature measurement signal (S1-S4, D1-D4) by means of a high-pass filter (HP) and subsequently to detect the signal fluctuations in the respective temperature measurement signal (S1-S4, D1-D4).

5. Fire detector according to one of claims 1 to 3, wherein the control unit (MC) is configured to first filter the respective detected temperature measurement signal (S1-S4, D1-D4) by means of a high-pass filter (HP), a high-pass filter signal (F) output by the respective high-pass filter (HP) H I-FJM) using a low-pass filter (TP) with a filter time (FZ) in the range of 1 to 120 minutes, preferably of 15 to 60 minutes, to filter and output as a respective filter output signal (F01-F04), and wherein the control unit (MC) is configured to output the warning message (WM) if the magnitude of at least one, preferably all, of the filter output signals (F01-F04) falls below a predetermined limit value (GW) of 5 mK, in particular 3 mK, preferably 1.5 mK, instead of the signal bandwidth of the signal fluctuations of the respective temperature measurement signal (TI— T4), for a minimum time (MZ).

6. Fire detector according to one of claims 1 to 3, wherein the control unit (MC) is configured to first filter the respective detected temperature measurement signal (S1-S4, D1-D4) by means of a high-pass filter (HP), a high-pass filter signal (F) output by the respective high-pass filter H iF H4) to filter by means of a moving average filter, in particular by means of a moving square or arithmetic mean filter (RMS, AVS), with a predetermined filter time (FZ) in the range of 1 to 120 minutes, preferably from 15 to 60 minutes, and to output as a respective filter output signal (F01-F04), and wherein the control unit (MC) is configured to output the warning message (WM) if the magnitude of at least one, preferably all, filter output signals (F01-F04) exceeds a certain threshold for a minimum time (MZ). specified limit value (GW) of 5 mK, in particular of 3 mK, preferably of 1.5 mK, instead of the mean signal bandwidth of the signal fluctuations in the respective temperature measurement signal (T1-T4).

7. Fire detector according to claim 5 or 6, wherein the specified limit value (GW) is set in a range of 1.5 to 10 times, in particular 1.5 to 5 times, a reference value which has been determined metrologically on a new, as-new or overhauled, in particular brand-new fire detector (M) in a flow-free test environment.

8. Fire detector according to one of claims 3 to 7, wherein the control unit (MC) is configured to continuously convert the respective temperature measurement signal (S1-S4) into the respective oversampled digital temperature measurement signal (D1-D4) by means of the A / D converter (ADC) by a number of OVS of oversampling in a range of 2 to 4096, in particular with a number of OVS of 64, 256 or 1024.

9. Fire detector according to one of the preceding claims, wherein the control unit (MC) is configured to control the respective thermistor (T1-T4) such that it heats up by less than 0.5 mK, in particular less than 0.1 mK, per measurement acquisition, and / or that it absorbs a maximum electrical energy of 10 pj, in particular 5 pj, preferably 0.5 pj, per measurement acquisition.

10. Fire detector according to one of the preceding claims, wherein the respective thermistor (T1-T4) has a maximum thermal response time of 3 s, in particular of 2 s, preferably of 1 s.

11. Fire alarm according to any of the preceding claims, - wherein in the housing (G) a communication with the ambient air via at least one inlet opening (OE) Fire detection unit (MK), in particular an optical one Measuring chamber, designed or installed to record at least one fire characteristic parameter, - wherein a grid (N), in particular an insect screen, is arranged in the area of ​​the respective entry opening (OE) and the fire detection unit (MK), and - wherein the grid (N) is arranged between the at least one thermistor (T1-T4) and the fire detection unit (MK) in the area of ​​the respective inlet opening (OE), or - wherein at least one thermistor (T1-T4) is arranged between the grid (N) and the fire detection unit (MK) in the area of ​​the respective entry opening (OE).

12. Fire detector according to one of the preceding claims, wherein the control unit (MC) is configured to start an operating timer (BZM) of the fire detector (M) at the start of the electrical power supply to the fire detector (M), wherein the control unit (MC) is configured to output a covered message (COV), or the warning message (WM) together with a covered message (COV), as an indication of an impermissible covering of the at least one inlet opening (OE), if the signal bandwidth of signal fluctuations of at least one of the detected, preferably all, temperature measurement signals (Sl-S4) is in a range of a minimum waiting time (WZ) for a minimum waiting time (WZ). for 1 hour to 3 days, preferably from 1 hour to 24 hours, a value of less than 30 mK, in particular less than 20 mK, preferably less than 15 mK, and if, in addition, an operating time measurement value (BZW) of the operating time meter (BZM) is less than 2 years, preferably less than 1 year.

13. Fire detector according to one of the preceding claims, wherein the control unit (MC) is configured to output the warning message (WM) and / or the covered message (COV) optically and / or acoustically directly at the fire detector (M) and / or to a higher-level control unit connected to the fire detector (M) and / or wirelessly via a radio connection to a higher-level control unit.

14. Fire detector according to one of the preceding claims, wherein the control unit (MC) is configured to monitor the respective temperature measurement signal (S1-S4, D1-D4) for exceeding an overtemperature limit value of in particular 54 °C and / or a temperature increase rate of in particular 5°C per minute, preferably 10°C per minute, as fire characteristics and to issue a fire alarm (AL) in the event of a detected fire.

15. Method for detecting contamination or an impermissible covering of at least one inlet opening (OE) for smoke or combustion gas in a fire detector (M) , - wherein at least one unheated thermistor (T1-T4) is located in the area of ​​one of the inlet openings (OE), - wherein a temperature measurement signal (S1-S4) is detected by the respective thermistor (T1-T4), and - wherein a warning message (WM) is issued for the absence of moving ambient air around the at least one thermistor (T1-T4) when the signal bandwidth of signal fluctuations of at least one of the detected, preferably all, temperature measurement signals (S1-S4) falls below a value of 30 mK, in particular 20 mK, preferably 15 mK for a minimum time (MZ).

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